There is considerable interest today in optical power management in wavelength division multiplexed systems, particularly relating to addition and deletion of wavelengths (i.e. channels) in a network and suppression of resulting transient behavior. Resulting transient behavior is undesirable because it leads to an increase in error rate of data that is recovered by the receiver, and it can occur when the power per channel is either too high or too low. For example, if the power per channel is too high, then non-linear effects like self-phase modulation can increase. Further, if the power per channel is too low, then the signal to noise ratio is decreased. Thus, proper power management is critical to normal operation of a system, and methods have been developed in the past for achieving proper power management in wavelength division multiplexed systems.
Past systems have typically relied on using erbium doped fiber amplifiers that switch from an automatic power control mode to an automatic gain control mode when a ramp up or down in the input power (i.e. power ramp) occurs that results from respective addition or removal of a wavelength. According to this previous solution, switch in mode is triggered when a detected rate of change of input power exceeds a predetermined threshold, and the result is an alteration in control of the amplifier pump currents.
Both automatic power control and automatic gain control are achieved by controlling the erbium doped fiber amplifier pump currents. An automatic power control mode provides stability in that it achieves constant output power no matter how the input power to the amplifier may vary; thus this mode is preferable when wavelengths are not being added or removed along the path containing the amplifier. If wavelengths are added while maintaining automatic power control mode, however, power per channel can decrease. Similarly, if channels are removed while automatic power control mode is maintained, power per channel can increase. In either case, it is preferable to maintain a constant power per channel; thus, it is necessary for the amplifier to switch to an automatic gain control mode during addition and removal of wavelengths.
Automatic gain control mode achieves constant gain no matter how the input power may vary. As a result, adding and removing wavelengths while maintaining automatic gain control mode has no effect on the power per channel. It is still necessary, however, to switch back to automatic power control mode once the addition or removal is complete to maintain proper operation of the system. In the past-developed threshold-based solution, this switching back and forth between modes occurs automatically and is governed according to a predetermined threshold relating to rate of change of input power. Thus, when a wavelength is added or removed, the associated power ramp up or down is detected if it exceeds the predetermined threshold. In response, the amplifier can switch to automatic gain control mode for the duration of the power ramp and vary the output power to maintain the gain at its last known value. The threshold-based solution, however, has its weaknesses.
Overshoot and undershoot resulting from imperfect response time is one weakness of the threshold-based solution. Response time for a single amplifier may vary from one-hundred microseconds to one millisecond. Thus, by the time the amplifier switches to automatic gain control mode and back, an overshoot or undershoot of output power per channel occurs that is between approximately twenty-five one-hundredths decibels and three-tenths decibels. This weakness is particularly problematic in that response time and resulting overshoot or undershoot may vary depending on the number of wavelengths being added or deleted. In addition, response time and resulting overshoot or undershoot may vary from one set of wavelengths to another, even where the numbers of wavelengths in each set are equal. Moreover, the effects of undershoot and overshoot are cumulative between a transmitter and receiver. Thus, the threshold-based solution often proves inadequate, especially in ultra-long haul applications where fifty or more amplifiers may be employed between transmitters and receivers, and particularly when a large percentage of wavelengths are added or removed suddenly.
Another attempted solution to the problems relating to achieving proper power management in wavelength division multiplexed systems involves the use of dummy signals. This dummy signal-based solution provides power to unused bands to help provide stable power levels to the transmission system. Providing extra power when the actual channel count is low does succeed in providing extra stability even in ultra-long haul applications. This solution, however, has disadvantages.
There exist at least two disadvantages associated with the dummy signal-based solution. A first disadvantage of the dummy signal-based solution involves additional expense related to special circuit packs containing laser pumps that produce optical power to unused bands. A second disadvantage of the dummy signal-based solution stems from the fact that it is a manual process that is not suitable for automated setup and teardown in an agile optical network. These disadvantages, and especially the second disadvantage, render the dummy signal-based solution unsuitable as a total solution with next generation wave division multiplexed systems.
Reconfigurable optical communications systems exhibit capabilities that are inhibited by application of the threshold-based solution and the dummy signal-based solution, even when the two solutions are used together in the most beneficial fashion. Capabilities exhibited by these systems include an increase in the number of wavelengths per band, and an increase in the distance a wavelength can travel (i.e. ultra-long haul). These capabilities further include automatic and/or manual optical switching of wavelengths and/or bands of wavelengths between line systems, and mesh restoration techniques involving automatic rerouting of channels around points of failure in the network. Still further, these capabilities include dynamic addition and deletion of wavelengths as demand increases and decreases, respectively. Overall, these capabilities involve rapid addition and removal of wavelengths at any point in the network, and those points frequently correspond to ultra-long haul line systems carrying many wavelengths per band. The threshold-based solution cannot provide sufficient power management at those points, while the dummy wavelength-based solution inhibits rapid addition and removal of wavelengths. Thus, there remains a need for a solution that provides sufficient power management in next generation systems, and providing such a solution remains the task of the present invention.